Hard Disk Drive Assemblies (HDA) are sensitive devices that are used by computer systems for magnetically storing and retrieving data. Normally, disk drives are subject to linear and/or rotational, external and/or internal, small and/or large shocks or accelerations. A typical external shock is one produced by the movement of the table on which the computer system rests. A typical internal shock is one generated by the reaction to the motion of the magnetic heads and associated devices during positioning operations. Such internal or external shocks cause the disk drive system to vibrate and may shift the magnetic heads off track. In addition to shifting the magnetic heads off track, certain external shocks could damage the HDA. Therefore, it is desirable to design and install a disk drive shock absorption and protection mechanism to (1) protect the disk drive against potential damage by such shocks, and (2) apply signals to prevent the magnetic heads from being moved off the desired track by such accelerations. At best, these goals have only been partially satisfied by prior art methods. More specifically, the prior art employs certain types of (1) shock-mounts supporting HDA systems to absorb external shocks, and (2) servo control circuitry to detect and subsequently compensate for the shift in the position of the heads resulting from certain shocks.
The shock mounts are normally made of rubber, having a pretermined stiffness. The stiffness of the shock-mounts and such physical characteristics as the weight of the HDA determine the natural frequency of oscillation of the HDA. The stiffer the shock mount, the higher the natural frequency of oscillation of the disk drive. Normally, this natural frequency is less than 1000 Hertz. Unfortunately, however, a shock mount that is too stiff fails to adequately protect the HDA against potential damage by external shocks. Further, if the shock mount is too soft or flexible, it will not adequately resist acceleration or movement caused by reaction to magnetic head repositioning. Accordingly, a compromise is normally selected, with the frequency of oscillation of the assembly and shock mounts being in the order of 100 to 200 Hertz. Regardless of the stiffness of the shock-mounts, higher amplitude shocks will normally cause the HDA to vibrate and move the heads off track. To detect and compensate for such displacement of the heads, servo control circuitry is employed to keep track of the position of the magnetic heads. To gain an understanding of how the servo control circuitry operates, a brief explanation of the operation of a typical disk drive is made below.
A disk drive normally includes, among other elements, a stack of spaced magnetic disks for recording data, a plurality of magnetic heads for performing input/output operation on the magnetic disks, with at least one of the magnetic heads being used as a servo head, and servo control circuitry for compensating for the shifts in the location of the servo head, and a head-positioning coil for controlling the movement of the heads.
Normally, the magnetic disks include a predetermined number of tracks for recording and retrieving data. Further, a track on a particular disk is normally grouped with the corresponding tracks on the rest of the disks. Such a grouping of the tracks constitutes a logical "cylinder". The tracks that form the logical cylinder are normally parallel with each other, because they are located on disks that are parallel and co-axial with each other. Further, the magnetic heads are stacked parallel to each other and move in parallel. Thus, if a particular magnetic head is located on a particular track in a cylinder, the rest of the magnetic heads will also be located on the corresponding parallel tracks in the cylinder. In this manner, all of the magnetic heads are moved in parallel and simultaneously over their respective tracks.
Therefore, any acceleration that causes one of the magnetic heads to move off the desired track, similarly causes the rest of the magnetic heads to be shifted off their respective desired tracks by the same amount. Thus, by adjusting the position of one of the magnetic heads, all of the magnetic heads will be adjusted simultaneously.
Normally, the particular head that is used for detecting such displacements of the magnetic heads is called a servo head. Further, the circuitry that provides the necessary signal to move the heads back on track is called the servo control circuitry, or servo loop. The servo loop supplies this signal to the head-positioning coil, which applies a corresponding acceleration to the head positioning assembly carrying the magnetic heads.
The servo circuitry uses certain pulses known as servo pulses to detect the displacement of the servo head off its intended track. These servo pulses are recorded on at least one of the magnetic disks. The servo head, which is in parallel with the rest of the magnetic heads, reads the servo pulses and transmits them to the servo circuitry. The servo circuitry uses this data to detect whether the servo head is off track. If the servo head has moved off the desired track, the servo circuitry provides the appropriate signal to the magnetic coil to move the heads back on track. Unfortunately, however, the servo circuitry is only capable of compensating offsets in the position of the servo head resulting from application of certain accelerations. For example, the servo circuitry is not capable of countering the effect of large rotational accelerations in an HDA system that uses a head-positioning assembly that rotates about an axis. In such systems, rotational accelerations normally may cause an error in the input/output operation of the head, and/or a relatively longer delay in the input/output operation.
The capability of the servo circuitry to resist such accelerations applied to the system is also known as the stiffness of the circuit. The stiffer the circuit, the greater is its capability to resist such accelerations. However, the servo circuitry becomes unstable if its stiffness is increased beyond a certain limit. Therefore, when large rotational accelerations are applied to the system, they normally cause a delay and/or error in the input/output operation of the HDA system.
Previous proposals directed to this type of problem include D. W. Rickert U.S. Pat. No. 4,477,755, issued Oct. 16, 1984, and an article entitled "Design Strategies for High-Performance Incremental Servos" by Martyn A. Lewis, Proceedings of the Sixth Annual Symposium on Incremental Motor Control Systems and Devices, May 1977, Department of Electrical Engineering, U. of Ill., Urbana Champagne, Ill. The Rickert patent discloses an electrical model of the mechanical system of a disk drive to respond to internally generated vibrations resulting from certain seek signals, and circuitry to compensate for the expected vibrations. The article discloses the use of "Transducers for External Vibrations" and suggests "measurement of external disturbances and applying these signals (via appropriate networks) to the summing junction so as to cancel the effects of external disturbances." However, the nature of the proposed transducers or disturbances and specifics for implementation of the system are not disclosed.
Accordingly, the primary object of this invention is to provide an inexpensive shock resistance mechanism for the HDA that provides the head-positioning coil with the necessary signal to resist large or small, internal or external, rotational or linear accelerations, and thereby (1) increase the speed of the input/output operation even when the system is subject to such accelerations, (2) minimize the possibility of input/output error when the system is subject to such accelerations, (3) maintain stability of the servo circuitry, and (4) provide for the possibility of using softer shockmounts that better protect the HDA against potential damage by relatively large external shocks.